Drilling is a process that is generally used to cut an aperture into an article, and tapping is a process that is generally used to form or cut a thread inside of an aperture.
Conventional drilling or tapping processes are relatively slow for forming apertures in articles. Stamping or forging processes offer a quicker process then drilling, however, such processes may result in less precise apertures. Stamping or forging a thread in an aperture may not be feasible depending, at least in part, on the condition of the aperture and/or the precision and surface quality of the aperture walls.
Furthermore, some metals are soft, chemically active, and have a strong tendency to adhere to tool surfaces during machining (e.g., aperture formation). Diamond coatings have been applied to tools to provide a hard surface with low friction. Such coatings are chemically inert, which substantially prevents the metals (e.g., the workpiece) from sticking to the tool.
One potential problem with diamond coatings is that the residual compressive stress may increase in the coating as the article cools from the deposition temperature to room temperature. Such stress may, in some instances, result in the diamond coating delaminating from the substrate material. Such residual stress may develop as a result of the large coefficient of thermal expansion (CTE) mismatch between the diamond coating and the substrate material.